The invention relates to a method of manufacturing an optical fiber preform in an installation for manufacturing or building up preforms having supporting cores. Such an installation includes rotation means having a horizontal axis of rotation and two mounting points between which the supporting core of the preform to be manufactured or built up is mounted, plasma-torch and material-supply means disposed radially relative to said supporting core and mounted to move in axial translation relative to and parallel to the supporting core so as to make said preform around said supporting core. The preform is manufactured or built up by effecting successive passes with the plasma torch while material is being supplied.
During the above step, a portion of the feed material that has not attached to the preform generates soot. In known manner, such installations are equipped with suction hoods which suck up the soot so as to prevent it from being deposited on the preform.
The manufacturing or building-up step is followed by a separation step in which the preform is cut transversely at one of its ends so that it can be removed from the installation.
During the separation step, the temperature of the separation zone of the preform is raised by means of the plasma torch or by means of a blow torch so as to make the separation zone ductile, and then the ductile separation zone is drawn until the preform is actually separated from its end-piece.
Apart from the soot generated by the material that has not attached to the preform, during the manufacturing or building-up step and during the separation step, the peripheral surface of the preform is heated by the plasma torch to a temperature such that the material making up the preform evaporates and condenses in the vicinity of said surface. The material evaporates, and then cools as it rises and condenses, thereby forming soot that falls back onto the preform.
Thus, during the last pass of the manufacturing or building-up step, soot is generated which, once deposited on the preform, significantly reduces the quality of the surface state of the preform. This results in an increase in roughness, and affects the transparency of the preform.
Likewise, during the separation step, heating the separation zone gives rise to soot deposition in the vicinity of the separation zone.
In order to remedy these drawbacks, an additional step is performed, whereby the preform is consolidated by means of a blow torch. This additional step significantly lengthens the preform manufacturing time because, in order to perform the additional step, the preform must have cooled sufficiently for it to be possible for an operator to consolidate the surface with a blow torch. Any reduction in cooling time considerably increases the risks of accident for the operator. However, locally re-heating a cooled preform during the surface consolidation step can have major consequences on the preform in the re-heated zone. In particular, cracks can occur. Therefore, a compromise must be struck between quality and safety because of the human presence during the vitrification stage.
In a method described in the Applicant's Document FR-A-2 730 505, after the plasma torch passes with material being supplied have been effected, and/or after the preform has been separated, at least one glazing step in which at least one glazing pass is effected automatically and without cooling the preform, which pass is effected with a plasma torch and without material being supplied so as consolidate deposits comprising condensation soot.
Such glazing, or heat polishing, makes it possible to eliminate imperfections (such as improperly melted grains).
In order to prevent soot from being re-deposited during such glazing (re-deposition of silica might pollute the fiber-drawing oven), a high glazing speed, approximately in the range 80 mm/min to 90 mm/min, should ideally be used. But such a glazing speed would give rise to unacceptable stresses in the preform, because the temperature in the preform would not be high enough to relax the stresses. Such a high speed would thus give rise to high risks of cracking or breaking, thereby reducing the life-span of the fiber.
Glazing is therefore generally performed at a mean speed of 40 mm/min, that speed being a compromise between the various requirements.
Unfortunately, although it is generally satisfactory, that prior art method still carries risks of re-deposition, in particular while the preform is being separated.